a) Field of the Invention
The present invention is directed to a rotary air coupling that provides a means to distribute air from an air supply to vehicle tires mounted on a rotating hub of a vehicle drive axle.
b) Description of Related Art
Automatic central tire inflation systems (CTIS) are becoming popular with the trucking industry. Various types of tire inflation system have been developed, both for trailer axles and tractor drive axles.
A central tire inflation system allows the operator to control the air pressure of the tires for improved performance of the tires and of the vehicle. However, the central tire inflation systems currently available suffer from two primary drawbacks. The axle and hub and possibly other parts must be extensively modified in order to accommodate a conventional system.
Central tire inflation systems in general are an important feature. Specifically, an off-road vehicle will often drive on soft, muddy surfaces and then switch to harder surfaces, such as paved roads. To obtain proper traction on soft, muddy surfaces, the tire should be operated at a relatively low pressure. To obtain proper traction and optimal fuel mileage on harder surfaces, the tires should be operated at relatively high pressures.
The differences between preferred high tire pressures (hard surfaces) and preferred low tire pressures (soft surfaces) may be quite large. For example, some truck tire manufacturers recommend a maximum tire pressure of 120 psi for highway conditions and 8 to 15 psi for extremely soft conditions such as sand, mud and swamps. A tire pressure for intermediate road conditions may be approximately 40 psi. Of course, in the case of trailers, the preferred tire pressure will also be dependent upon the weight of the goods or equipment carried in the trailer.
Another reason justifying the employment of central tire inflation systems is road surface deterioration. It is known that over-inflated tires are a major cause of road surface deterioration. Road repair and road building have become increasingly expensive and it is therefore foreseeable that governmental entities may begin penalizing the operators of trucks and tractor-trailers for operating equipment on public roads with improperly inflated tires or over-inflated tires.
Of course, another reason justifying central tire inflation systems is fuel economy. Soft tires on highways reduce gas or fuel mileage. A central tire inflation system that automatically controls the air pressures of trailer tires on a highway will increase fuel efficiency as well as improve road surface longevity.
Existing air distribution systems for tire inflation systems on drive axles direct the supply air through the bearings of the hub. Distribution through the bearings demand smaller bearings and require additional seals in the hub. The seal life is shortened due to the increased seal friction caused by pressure loading of the seal lips.
The need exists for a rotary air coupling designed to distribute supply air to the tires mounted on drive axles in which the air coupling is positioned externally of the drive axle and the hub bearings